MOSCOW — Russian prosecutors moved Friday to block calls on social networks for more street protests in Moscow and other Russian cities following a wave of rallies that have cast a new challenge to the Kremlin.

Tens of thousands demonstrators turned out in Moscow and other cities on Sunday to rally against official corruption in the largest outpouring of discontent in years.

The Prosecutor General’s office confirmed Friday it has requested the state communications watchdog to block pages on social networks calling for more protests in Moscow and elsewhere in Russia coming Sunday.

Russian opposition leader Alexei Navalny, who organized Sunday’s unsanctioned protest, is serving a 15-day sentence on charges of resisting police.

More than 1,000 protesters have been arrested in Moscow, and many of them also have been sentenced to brief jail terms and fines. A dozen people working for Navalny are among those serving jail time.

The U.S. and the EU have criticized the crackdown and urged Russia to release all detainees, but President Vladimir Putin has rejected the criticism as meddling in Russia’s affairs.

Without naming Navalny, Putin, who faces re-election in March 2018, has denounced protest organizers who try to use anti-corruption slogans in “narrow selfish political goals.”

Navalny has declared his intention to run for president and vowed to appeal a conviction that bars him from the race, which he denounced as politically-driven.